An implantable medical device is implanted in a patient to, among other things, monitor electrical activity of a heart and to deliver appropriate electrical and/or drug therapy, as required. Implantable medical devices (“IMDs”) include for example, pacemakers, cardioverters, defibrillators, implantable cardioverter defibrillators (“ICD”), and the like. The electrical therapy produced by an IMD may include, for example, pacing pulses, cardioverting pulses, and/or defibrillator pulses to reverse arrhythmias (e.g. tachycardias and bradycardias) or to stimulate the contraction of cardiac tissue (e.g. cardiac pacing) to return the heart to its normal sinus rhythm.
In general, the IMD includes a battery and electronic circuitry, such as a pulse generator and/or a processor module, that are hermetically sealed within a housing (generally referred to as the “can”). The housing typically is formed of titanium or other suitable corrosion-resistant, biocompatible, electrically conductive material. The housing includes opposed concave half shells that are welded together to form an interior cavity, in which the battery, pulse generator and/or processor module reside. The half shells have an oval contour with a header receptacle area configured to receive a header assembly that is joined to the device housing. A feedthrough assembly is located at the header receptacle area and forms an interface for conductive paths to enter/exit the interior cavity, while maintaining the hermetic seal. Conductor pins are inserted through the feedthrough to provide the conductive paths to and from electronic circuitry.
The header assembly holds a connector block that is configured to be joined to one or more leads that pass into the heart. One end of each lead is inserted into the connector block, while the other end of each lead includes an electrode that is to be positioned in an interior chamber of the heart. The electrode may deliver an electrical shock to defibrillate the heart, or the electrode may provide low energy signals to pace the heart. The electrode may be also used to sense cardiac electrical signals from the heart.
The leads transmit electrical signals between the heart and the pulse generator. In certain environments, a lead may effectively function as an antenna that is susceptible to external electromagnetic (EM) fields. When external electromagnetic fields become coupled to the lead or the connector block, electromagnetic interference (“EMI”) signals are generated and injected into the IMD. Patients are exposed to external electromagnetic interference sources in a variety of ways, such as in the home, workplace, public places, and medical facilities. Sources of EMI include cellular telephones (including communication bands for Bluetooth, HomeRF, and wireless LAN), anti-theft devices, keyless entry systems, medical equipment, microwave devices, welding equipment, sources of radio frequency interference (RFI), radio-controlled toys, television broadcast antenna, radio transmitters (e.g. broadcast and two-way), electronic article surveillance systems, and the like.
At specific frequencies, an electromagnetic field may electrically couple with the conductors within the leads, thereby producing an EMI signal that is conveyed along the conductors, through the connector block and feedthrough assembly to the electronic circuitry. The EMI signal may then interfere with the operation of the implanted medical device. For instance, the EMI signal may inhibit pacing or may cause fast, erratic pacing. Further, an EMI signal may resemble a cardiac signal from the heart, which could be misinterpreted by the pulse generator/processor as an event that requires therapy. In turn, the pulse generator may send a pacing or shocking pulse to the heart when not required. In addition, strong EMI signals may cause other problems with the internal electrical circuitry, such as to mistakenly indicate an end-of-battery life, inadvertently reset power-up conditions, cause over-sensing/under-sensing, or cause permanent damage to the circuitry.
To address the above concerns, EMI filter circuits have been designed that attenuate potential EMI signals before reaching the electrical circuitry within the implantable medical device. Conventional EMI filter circuits use “decoupling” capacitors along with surge protection blocks to prevent EMI from damaging the device. One conventional approach prevents EMI from entering the housing of the pulse generator by connecting a capacitor between each conductor pin connected to the sensing circuit or pulse generator and the device's case. Another existing EMI filter removes unwanted noise by shorting the high frequency signal path to the housing through a plurality of capacitors. Other EMI filters add a resistor or a lossy inductor along an energy delivery path, in series with a shunting capacitor.
However, EMI filters that include a series element are generally useful only within dedicated sensing paths or low voltage delivery paths, such as in pacemakers. Such EMI filters are not practical for use in high energy delivery paths, for example, as in defibrillators. Further, EMI filters that include a resistor or lossy inductor may be unable to withstand the high current and or high voltage present in a high-energy delivery path. Hence, a need exists for an EMI filter that is configured to operate in the high energy delivery path of an implantable medical device.